1. Field of the Invention
This invention pertains generally to circuit interrupters and, more particularly, to arc chutes for circuit breakers.
2. Background Information
Circuit breakers typically include a set of stationary electrical contacts and a set of moveable electrical contacts. The stationary and moveable contacts are in physical contact with one another when it is desired that the circuit breaker provide electricity therethrough to a load. When it is desired to interrupt the circuit, however, the moveable contacts are moved away from the stationary contacts, thus removing the moveable contacts from physical contact with the stationary contacts and creating a space therebetween.
The movement of the moveable contacts away from the stationary contacts results in the formation of an electrical arc in the space between the contacts beginning at the time the contacts are initially separated. Such an arc is undesirable for a number of reasons. For example, current flows through the circuit breaker to the load when it is desired that no such current should flow thereto. Additionally, the electrical arc extending between the contacts often results in vaporization or sublimation of the contact material itself, eventually resulting in destruction or pitting of the moveable and stationary contacts. It is thus desired to eliminate any such arcs as soon as possible upon their propagation.
The moveable contacts typically are mounted on arms that are contained in a pivoting assembly which pivots the moveable contacts away from the stationary contacts. An arc chute is provided along the path of each arm to break up and dissipate such arcs. Such arc chutes typically include a plurality of spaced apart arc plates mounted in a wrapper. As the moveable contact is moved away from the stationary contact, the moveable contact moves past the ends of the arc plates, with the arc being magnetically urged toward and between the arc plates. The arc plates are electrically insulated from one another such that the arc is broken up and extinguished by the arc plates. Examples of arc chutes are disclosed in U.S. Pat. Nos. 6,703,576; 6,297,465; 5,818,003; and 4,546,336.
U.S. Pat. No. 4,229,630 discloses deionization plates which may be utilized to direct the arc into the corners of each deionization plate so that the maximum length of the plate may be utilized for cooling and deionization of the resulting plasma. The plate has an opening therein which is generally in the shape of a V. However, the apex of the V-shaped opening is directed towards one of the frame sides of the arc chute. When inserted into the frame, these plates are positioned such that adjacent plates would have their apex directed to opposite side walls or, put another way, alternate plates would have their apex directed toward the same side.
U.S. Pat. No. 4,229,630 also discloses a pair of vertical arc gassing insulation plates secured to the deionization plate. The arc gassing insulation plates are disposed on opposite sides of the generally V-shaped opening and the arcing contact. The arc gassing insulation plates are made of a suitable arc gassing material, such as glass polyester or a ceramic-type material, and are inserted on either side of the arcing contact to increase the pressure at the arcing contact to drive the resulting arc more rapidly into the arcing chamber while concurrently allowing any arcs present at the main movable contacts to enter the arcing chamber.
Low voltage air circuit breakers interrupting relatively high currents (e.g., 100,000 A and higher) with molded housings and enclosed arc chambers may often sustain damage to their housings during short circuit interruption. Arcing energy at the corresponding power levels produces a pressure wave that may crack molded composite parts and collapse sheet-metal plates. Corresponding damage to the arc chute reduces its effectiveness, which increases arcing duration, energy release and chance of failure. In addition, residual ionized gas, with vaporized conductor material, may result in dielectric breakdown between the separable contacts even after the initial arc is gone.
Arc chutes are designed to encourage the arc to enter the metal arc plates. An arc can move quickly to the top edge of the arc plates and pass between top edges of some plates, thereby completely bypassing intermediate plates. This reduces the number of arc voltage drops and the effectiveness of the arc chute. This bypassing effect further creates current and gas flow patterns that tend to collapse groups of plates together, further reducing voltage divisions in the arc chute and its cooling effectiveness.
Another shortcoming of typical arc chute designs is that the gas flow from individual arc plate gaps recombines before exiting through the vent. This allows a few gaps that are directly above the center of the arc to dominate the gas flow. Relatively little gas flow (or arc mobility) occurs in the far forward or rearward plate gaps because they are competing with the central high-pressure gaps for exit flow area. The forward and rearward plates, and therefore the full volume of the arc chamber, are underutilized.
Retention of the arc chute top, even if it has a relatively large exit vent, is very difficult at interrupting currents above 100,000 A. The pressure wave may readily shatter a molded composite arc chute top and may pull fasteners through the molded material. Metal tops may emit unacceptable stray arc currents to the circuit breaker or enclosure ground. Metal arc chute tops may also attract arc from metal arc plates below, thereby conducting current in a manner that bypasses intermediate plates.
Accordingly, there is room for improvement in arc chutes and in circuit interrupters employing arc chutes.